In the normal process of walking, the human musculo-skeletal system undergoes shock as a result of the heel striking a hard surface. Modern medical research has linked the shock wave produced by the impact of the heel on a hard surface to a number of medical problems including, among others, migraine headaches, inner ear disturbances, lower back pain, osteoarthritis, heel pain, stress fractures, and tendinitis. See, for example, E. C. Frederick, "Bone Jolt", American Health, July/August 1982, p. 64; A. Voloshin and J. Wask, "An In Vivo Study of Low Back Pain and Shock Absorption in the Human Locomotor System", J. Biomechanics, Vol. 15, No. 1, 1982, p. 21; and Eric L. Radin et al, "Role of Mechanical Factors in Pathogenesis of Primary Osteoarthritis", The Lancet, Mar. 4, 1972, p. 519. Ordinarily, the body dissipates the shock wave by a series of shock-absorbing systems. The initial shock-absorbing mechanism is a half-inch thick mat of fat and connective tissue that makes up a pad under the heel. This pad absorbs an estimated 80% of the energy passing through it. The residual shock waves are dampened by the bones of the foot and lower leg and the surrounding fleshy tissue as the waves move up the musculo-skeletal system. The above-mentioned maladies may occur when there is a breakdown in a person's natural shock-absorbing systems.
Numerous orthotic devices have been proposed to prevent or alleviate the medical problems arising from a breakdown in the body's natural shock-absorbing systems. U.S. Pat. No. 4,346,525 discloses a cushion pad fabricated from a web of closed pore foam of cross-linked ethylene 1 vinyl acetate-low density polyethylene copolymer. The pad is formed between two opposing molds to present a shape having a cup-like depression adapted and constructed to accept essentially the heel portion of the wearer. A number of sizes are required to fit the feet of possible users.
U.S. Pat. No. 4,168,585 discloses a heel cushion formed of homogeneous elastomeric material adapted to conform to the insole of a wearer's shoe to relieve the pain of a heel spur. The cushion tapers toward its forward end and has an elongated cavity in the lower surface thereof extending directly under the heel bone and a slight concavity in the upper surface located over the cavity.
U.S. Pat. No. 4,120,102 discloses an insert to fit inside a shoe below the heel of the foot. The insert is a pad of resilient material such as artificial rubber having a smooth top surface and an under surface with a plurality of ribs extending radially out from a center area under the middle of the heel. The ribs are shaped and disposed such that the pressure of the heel and the shape of the ribs tends to rotate or twist the heel around and away from the other foot.
U.S. Pat. No. 4,325,380 discloses an orthotic device having a cupshaped body member adapted to surround the heel and adjacent portions of a person's foot. The body member has three substantially solid rib members extending longitudinally along the length of the bottom of the body member. Each rib member has a width greater than its height.
U.S. Pat. No. 4,619,056 discloses an insole including a cushioning layer having a perimeter which generally conforms to the contour of the sole of the foot. On the bottom surface of the mid portion of the insole, a plurality of arcuate ridges are disposed extending across the entire width of the insole to form arcs of concentric circles having their radial centers in the middle of the heel portion of the insole.
U.S. Pat. No. 4,268,980 discloses a shock-absorbing, relatively rigid but resilient slab for insertion into footwear. The slab extends across the shoe width and lengthwise from the rear border of the heel to the metatarsal heads of the foot. The slab has a smooth top surface with a heel concavity and a varus heel ridge extending transversely from the lateral to the medial side and inclined upward to a high level at the medial side and then continuing medially to the rear end of the heel cup. The bottom has a solid lateral edge from which a plurality of longitudinally spaced ribs radiate circumferentially in a clockwise direction around the heel for an arc of about 30 degrees and then along the medial edge.
U.S. Pat. No. 4,510,699 discloses an insole of wedge-shaped, irregular or curved section fabricated from a composition of silicone rubber, inorganic filler, hydrosilicate powder and vulcanizing agent. The composition is mixed to obtain a slurry of high fluidity, poured into a mold and then heated in an oven for about an hour to harden.
One of the problems with many of the devices of the prior art is the need to provide a wide variety of sizes to fit the feet and shoes of different wearers. This creates the need for having multiple sets of molds and dies for manufacturing different sizes, which adds to the expense of producing the device for commercial sale.
Other problems arise from the materials used to make the devices. While foamed or sponge materials tend to relieve the pressure generated by the impact of the heel, they lack the resiliency needed for long-term use. For example, many foams lose their compressibility after continued use and form impressions of the foot or other high pressure areas, because of their creep properties. On the other hand, more resilient materials are often difficult to foam or are unable to be foamed and lack the compressibility needed to provide relief from painful pressure.